Washington  The Supreme Court rebuked President Bush and his anti-terror policies Thursday, ruling that his plan to try Guantanamo Bay detainees in military tribunals violates U.S. and international law.

The president and congressional Republicans immediately pledged to work on a new strategy for special trials for some of the hundreds of suspected al-Qaida and Taliban operatives rounded up in Afghanistan, Pakistan and other countries.

Bush said the ruling "won't cause killers to be put out on the street."

The court declared 5-3 that the president's attempt to resurrect a type of military trial last used in the aftermath of World War II violates U.S. military law and the Geneva Conventions that set international standards for dealing with people captured in armed conflicts.

The ruling focused on Salim Ahmed Hamdan, a one-time driver for Osama bin Laden who has spent four years in the U.S. prison at Guantanamo Bay, Cuba. He faces a single count of conspiring to commit terrorism.

Lt. Cmdr. Charles Swift, Hamdan's Navy lawyer, said he told the Yemeni about the ruling by telephone.

"I think he was awe-struck that the court would rule for him, and give a little man like him an equal chance. Where he's from, that is not true," Swift said.

The decision could have a broad impact on the administration's legal justification for many of its policies in the global fight against terrorism, from eavesdropping to detention policies in Iraq.

No 'blank check'

The ruling united the four most liberal justices with moderate Anthony Kennedy in an opinion that showed the high court would not watch the controversy over Bush policies from the sidelines.

It was a sequel to a ruling two years ago that found the administration did not have a "blank check" to lock up alleged combatants without any legal rights. Again, the court said the Bush administration had gone too far.

"The Constitution is best preserved by reliance on standards tested over time and insulated from the pressures of the moment," Kennedy wrote in one opinion.

The ruling came on the court's final day before the justices began a three-month break. Court members spent more than half an hour announcing the decision and reading dissents.

Chief Justice John Roberts was sidelined in the case because as an appeals court judge he had backed the government in this case last year. That ruling was overturned Thursday.

The other three conservative justices, Antonin Scalia, Clarence Thomas and Samuel Alito, strongly supported the government.

"It is not clear where the court derives the authority - or the audacity - to contradict" Congress and the executive branch, Scalia wrote.

Thomas, reading a dissent from the bench for only the second time in his nearly 15-year career, said the court's decision would "sorely hamper the president's ability to confront and defeat a new and deadly enemy."

Difficult to defend

Hamdan's attorneys, backed by lawyer groups and military experts, had argued that the tribunals, before as few as three military officers, made it difficult for suspects to defend themselves, including confronting accusers and being present for all parts of a trial.

The majority found that Congress had not given Bush the authority to create the special type of military trial and that the president did not provide a valid reason for the new system. The justices also said the proposed trials did not provide for minimum legal protections under international law.

While the administration could come up with a new system, a better option would be to hold regular military courts-martial for detainees, the high court said. Those trials, used for soldiers, provide somewhat similar legal protections to those that defendants receive in U.S. courts.

The Bush administration did not appear ready to accept that.

White House counselor Dan Bartlett said the administration's task now was to determine how to design military tribunals that would pass muster. Bartlett said Bush could portray any lawmaker who objected to legislation as supporting the release of dangerous terrorists.

Late Thursday, Sen. Arlen Specter, R-Pa., filed legislation authorizing the president to create military tribunals and providing due process guidelines for trials of terrorist combatants. The Senate Armed Services Committee and Senate Judiciary Committee plan hearings during the summer.

The ruling said nothing about whether the administration should close the detention facility at Guantanamo Bay, as Bush has said he would like to do eventually. The administration erected the prison in the months after the attacks of Sept. 11, 2001, and it has been a flashpoint for international criticism.

For now, there are about 450 detainees at Guantanamo, and 115 of them have been deemed eligible for release or transfer to their home countries. Ten, including Hamdan, have been charged with crimes. The chief Guantanamo prosecutor had said about 65 more detainees were likely to be charged if the court ruled differently.